970 



THE ORGANS OF DIGESTION. 



districts in the whole canal, mouth, throat and visceral clefts, oesophagus, spindle- 

 shaped stomach, and the remaining tube connected with the yolk-sac. 



By the fifth or sixth week, the stomach, at first straight and parallel with the 

 axis of the body, begins to show a convex greater curvature toward the vertebral 



column and a concave lesser curvature on the opposite 

 side. This is covered by the voluminous liver. The 

 pyloric end is tilted away from the column and this 

 forms the duodenal loop (Fig. 588). As the commu- 

 nication with the yolk-sac becomes constricted and 

 absorbed there is developed a long umbilical loop of 

 intestine opposite the vitelline duct. The end of this 

 loop passes for a time into the umbilical cord sur- 

 rounded by a protrusion of the peritoneum. This 

 loop passes downward and forward, and consists of 

 two nearly parallel arms between which is stretched 

 the mesentery. At a little distance from the vitelline 

 duct on the lower arm of the loop is seen a small en- 

 largement ; this marks the future ccecum and begin- 

 ning of the colon and end of small intestine. Five 

 portions of intestine are now to be distinguished: 

 the short part passing back from the stomach toward 

 the spinal column becomes duodenum ; at the turn- 

 ing-point into the umbilical loop is the duodeno-jejunal 

 junction ; the anterior descending arm and bend of 

 the loop become small intestine ; most of the ascending posterior arm becomes 

 colon, and the terminal part, sigmoid flexure and rectum. 



At first the alimentary tract is mostly in contact with the chorda dorsal is 

 held by a broad mass of embryonal connective tissue. This tissue contains two 



, tenor 



Mesentery 



' Umbilicus 

 Alimentary Tract 

 Posterior Mesentery 



Aorta 

 Chorda Dorsalis 



FIG. 589. Fig. 586 with mesen- 

 teries added. (Dexter.) 



Spleen 



Lesser Curvature 

 'renter Curvature 

 Mesogastrium 



'esentery 



Colon 



Vitelline 

 Duct 



FIG. 590. Diagram to show the original positions of the liver, stomach, duodenum, pancreas and spleen, 

 and the mesenteries connected. A longitudinal section. (Hertwig.) 



primitive aortae. The right and left portions of the body-cavity approach each 

 other and compress this tissue into a mesentery, which is attached to the whole 

 length of the tube beginning with the stomach, connecting it with the chorda. 

 The special part of this membrane attached to the stomach posteriorly is called 

 mesogastrium, that to the small intestine is called mesentery (medium intestinum 



